This invention relates to the application of coatings to substrates and measurements of the thickness and/or the dry weight of the coatings on the substrates. This invention also relates to controlling the application of coating materials to substrates.
This invention is particularly useful for measuring wet clay coatings which are applied to paper or paste-board or box-board substrates to provide a smooth white printable surface on the substrates. In order to properly control the application of clay coatings to the paste-board substrate, it is desirable to obtain some sort of measurement on the coating material which has been applied to previously coated substrates so as to correct for any undesirable deviations on subsequently coated substrates. More particularly, it is desirable to obtain thickness and/or dry weight measurements of the coatings on substrates so as to properly control the application of coatings to other substrates.
Prior art nuclear absorption techniques for performing these measurements on coatings are not well adapted for use in this application since these techniques will not distinguish between a variation in the thickness of the substrate and a variation in the thickness of the coating material. Since the coating may only be a few thousandths of an inch thick, and the paste-board substrate may be as much or more than forty times thicker than the coating, any slight variation in the paste-board substrate will result in very serious errors in the measurement of the coating thickness if the prior art nuclear techniques are utilized.
It has been recognized that the thickness of a relatively conductive film on an insulating substrate may be measured by determining the conductance of the coated substrate since fluctuations in the thickness of the substrate will have little or no effect on the measured conductivity. Such an arrangement is shown in U.S. Pat. No. 3,043,993 wherein the amount of glue on paper is measured utilizing two plate electrodes in contact with the side of the paper opposite the side on which the glue is applied. However, the actual conductance which is measured varies as a function of temperature and can lead to erroneous indications that the amount of glue has varied when in fact only the temperature has varied.
U.S. Pat. No. 3,491,595 - Griffeth discloses a system for measuring the conductivity of soil to determine the moisture content of the soil utilizing a temperature sensing element to compensate for variations in temperatures sensed by a soil probe. If such compensation were utilized with the system described in my U.S. Pat. No. 3,043,993, the system would provide an indication of the overall conductance of the conductive coating but would not provide an indication of the thickness of the coating.